This invention relates to reconstructed joints between bones and, in particular, to an improved prosthetic joint having a mass of expanded PTFE as the articulating, connective material.
In current medical practice, a joint severely afflicted with disease or injury is replaced with one of a variety of prostheses. One type of prosthesis includes metal socket elements which are shaped similarly to the adjoining ends of healthy bones and have stems for insertion into bones. This type of prosthesis is used for a joint subjected to heavy loads, such as the hip joint. Another type of prosthesis, typically used for replacing damaged joints between bones subjected to minimal loads, includes an elastomer hinge for coupling the bones. This type of prosthesis is less expensive than the first type and is more suited for smaller joints, e.g. the metacarpophalangeal (MCP) joints in the second through fifth fingers.
It is desired to restore the integrity and functionality of a joint as much as possible. For joints normally having limited motion, this goal is relatively easily attained. For joints capable complicated motions, such as the bi-axial motion of the fingers, this goal becomes far more difficult. The bi-axial motion of a thumb is readily demonstrated by placing a hand flat upon a flat surface. Moving the thumb toward the palm (adduction) or away from the palm (abduction) while the thumb rests on the surface is motion about one axis and moving the thumb up from the surface (extension) or into the surface (flexion) is motion about the second axis.
Movement about two axes can be achieved with mechanical prostheses but these are often too expensive and of limited reliability. Prostheses made with elastomer hinges either do not provide bi-axial motion or permit unnatural motion of the bones. Either type of prosthesis in the prior art provides a hinge or swinging kind of motion, not the sliding motion of a natural joint.
Mechanical, i.e. metal, joints are often fastened to the bones with a bone adhesive. However, the mechanical joints are not resilient and the shock transmitted to the joint often causes the adhesive layer to crack or to remove a thin, adherent layer of bone tissue. In addition, the bone adhesive deteriorates with time. As a result, the joint must eventually be repaired or replaced.
Elastomer hinges have the advantage of being resilient but are rapidly falling into disfavor because they are typically made from silicone, which has been found to be incompatible with the tissue surrounding an implant. Silicone can break down into small particles that are ingested by body cells, causing an inflammation known as "silicone synovitis". The flexible web connecting the two halves of the hinge can fracture from repeated use and the whole prosthesis may have to be replaced. Also, silicone in prolonged contact with bone can cause the bone to dissolve.
All known prostheses currently used have limited life in the patient before the problems described above begin to appear, sometimes as soon as one year after a joint is replaced. A problem that occurs immediately upon replacement of a joint is the body's healing process which fills a joint with fluid. Drains to reduce swelling can help but therapy must start as soon after surgery as possible to minimize formation of scar tissue which can freeze a joint.
Polytetrafluoroethylene (PTFE) is known to be chemically stable and bio-compatible and expanded PTFE has been used in either tubular or sheet form for vascular grafts. In non-medical applications, expanded PTFE is used as a gasket material for example.
As disclosed in U.S. Pat. 4,187,390 (Gore), expanded PTFE is typically made by a cold extrusion process in which a paste of PTFE and lubricant is forced through a die. The extruded PTFE expands and is kept expanded during "sintering," i.e. heating the PTFE almost to its melting point, approximately 340.degree. C., and then allowing the PTFE to cool. After sintering, the extrusion is cut into sections and is ready for use. If squeezed between the fingers and released, expanded PTFE compresses and remains in its new shape until extended.
Expanded PTFE has a microscopic structure of nodes interconnected by fibrils and is porous. Porosity is measured as the average distance, e.g. 8-10 microns, from one node to another among a plurality of nodes making up a pore, i.e. porosity is a measure of fibril length. The elastic deformation of millions of tiny fibrils permits the expanded PTFE to compress or stretch and return to its original shape.
In view of the foregoing, it is therefore an object of the invention to provide a joint prosthesis having a longer life in the patient and improved function.
Another object of the invention is to provide a joint prosthesis having a more natural function.
A further object of the invention is to provide a simple joint prosthesis which can move in two axes.
Another object of the invention is to provide a joint prosthesis in which the cushion is preformed with stems for controlling the motion of the joint and enhancing the stability of the joint.
A further object of the invention is to provide a joint prosthesis having high damping capability.
Another object of the invention is to provide a joint prosthesis that is bio-compatible with a patient.
A further object of the invention is to provide a joint prosthesis having controlled adherence to surrounding tissue.
Another object of the invention is to simplify the installation of joint prostheses.
A further object of the invention is to provide a joint prosthesis which cannot fill with body fluids.
Another object of the invention is to provide a prosthesis that is less expensive than prostheses of the prior art while providing the same function.